Chapter 18: Reaction Rates and Equilibrium 18.1 Rates of Reaction

18.1 Collision Theory In chemistry, the rate of chemical change, or the reaction rate, is usually expressed as the amount of reactant changing per unit time.

Collision Theory 18.1 A rate is a measure of the speed of any change that occurs within an interval of time. Speed is measured as a change in distance in a given interval of time. A world-class sprinter might cover 100 meters in 11.5 seconds; the speed, or rate, is 100 m/11.5 s = 8.70 m/s.

Average Reaction Rate Equation Avg rxn rate = -Δ[reactant] Δt Δ[reactant] is the change in concentration of reactant Δt is the change in time

Example In a reaction between butyl chloride (C4H9Cl) and water, the concentration of C4H9Cl is 0.220 M at the beginning of the reaction. At 4.00 s, the concentration is 0.100 M. Calculate the average reaction rate.

Practice For the reaction H2 + Cl2 → 2 HCl Time (s) [H2] (M) [Cl2] (M) [HCl] (M) 0.00 0.030 0.050 0.000 4.00 0.020 0.040 For the reaction H2 + Cl2 → 2 HCl Calculate the average reaction rate with respect to H2; with respect to Cl2 If the average reaction rate is 0.0050 mol/L·s HCl, what concentration of HCl would be present after 4.00 s?

18.1 Collision Theory According to collision theory, atoms, ions, and molecules can react to form products when they collide with one another, provided that the colliding particles have enough kinetic energy.

18.1 The minimum energy that colliding particles must have in order to react is called the activation energy. Collision Theory The activation-energy barrier must be crossed before reactants are converted to products. INTERPRETING GRAPHS a. Navigate Which are at a higher energy, the reactants or products? b. Read Is energy absorbed or released in progressing from the reactants to the activated complex? c. Interpret Once the activated complex is formed, will it always proceed to form products? Explain.

18.1 Collision Theory An activated complex is an unstable arrangement of atoms that forms momentarily at the peak of the activation-energy barrier. The activated complex is sometimes called the transition state.

Factors Affecting Reaction Rates 18.1 Factors Affecting Reaction Rates The rate of a chemical reaction depends upon temperature, concentration, particle size, and the use of a catalyst.

Increasing temperature increases reaction rate 18.1 Temperature Storing foods in a refrigerator keeps them fresh longer. Low temperatures slow microbial action. Increasing temperature increases reaction rate Refrigeration Storing foods in a refrigerator keeps them fresh longer. Low temperatures slow microbial action. Meat kept at freezing temperatures has a lifetime of months.

Concentration Increasing concentration increases the reaction rate

Particle Size A smaller particle size (greater surface area) increases the reaction rate

Catalysts Increase the reaction rate by lowering the activation energy

18.1 An inhibitor is a substance that interferes with the action of a catalyst. Antioxidants and antimicrobials used in drying fruits and preserving fruit juices slow the action of microbes and limit contact with air.

18.1 Section Quiz. 1. The units below that would be appropriate to measure the rate of a chemical reaction is a) mol/s. b) mol/L. c) kJ/mol. d) h/mol.

18.1 Section Quiz. 2. In a chemical reaction, the energy of reactants is always a) greater than the energy of the products. b) more than the activation energy. c) less than the activation energy. d) less than the energy of the products.

18.1 Section Quiz. 3. An increase in which one of the following will NOT increase the reaction rate? a) temperature b) concentration of reactants c) total mass of reactants d) surface area of reactants

18.1 Section Quiz. 4. A catalyst works because it a) lowers the activation energy. b) increases the temperature. c) is permanently changed in a reaction. d) supplies energy to a reaction.

18.2 Reversible Reactions and Equlibrium

18.2 Reversible Reactions A reversible reaction is one in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously.

18.2 Reversible Reactions When the rates of the forward and reverse reactions are equal, the reaction has reached a state of balance called chemical equilibrium. The relative concentrations of the reactants and products at equilibrium constitute the equilibrium position of a reaction.

Factors Affecting Equilibrium: Le Châtelier’s Principle 18.2 Factors Affecting Equilibrium: Le Châtelier’s Principle The French chemist Le Châtelier proposed what has come to be called Le Châtelier’s principle: If a stress is applied to a system in dynamic equilibrium, the system changes in a way that relieves the stress.

Concentration If you increase the concentration of reactants, you make more products (shift right) If you add more product, you make more reactant (shift left) If you remove product, you make more product (shift right)

Temperature An increase in temperature shifts in the direction that absorbs heat Ex: in an exothermic reaction heat is a product so increases temp. forms reactants; removing heat would shift right to form products

Pressure An increase in pressure shifts to the side with fewer moles of gas

Conceptual Problem 18.1

for Conceptual Problem 18.1

N2O4(g) + 58 kJ ↔ 2NO2(g) Addition of heat Decrease in pressure Addition of NO2 Removal of N2O4

Equilibrium Constants 18.2 Equilibrium Constants The equilibrium constant (Keq) is the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to a power equal to the number of moles of that substance in the balanced chemical equation.

Equilibrium Constants 18.2 Equilibrium Constants A value of Keq greater than 1 means that products are favored over reactants; a value of Keq less than 1 means that reactants are favored over products.

Write Equilibrium Expressions for: H2(g) + I2(g) ↔ 2 HI(g) N2(g)+O2(g) ↔ 2 NO(g) 2 SO3(g) ↔ 2 SO2(g)+O2(g)

Practice Problem 18.1

for Sample Problem 18.1

Practice Problem 18.2

Practice The reaction COCl2(g) ↔ CO(g) + Cl2(g) reaches equilibrium at 900 K. Keq is 8.2 x 10-2 . If the equilibrium concentrations of CO and Cl2 are 0.150 M, what is the equilibrium concentration of COCl2?

Practice At 1405 K, hydrogen sulfide decomposes to form hydrogen gas and diatomic sulfur. The equilibrium constant for the reaction is 2.27 x 10-3. What is the concentration of hydrogen gas if [S2] = 0.0540 mol/L and [H2S] = 0.184 mol/L?

Practice Keq = 10.5 for the equilibrium CO(g) + 2H2(g) ↔ CH3OH(g) A) Calculate [CO] when [H2] = 0.933 M and [CH3OH] = 1.32 M B) Calculate [H2] when [CO] = 1.09 M and [CH3OH] = 0.325 M C) Calculate [CH3OH] when [H2] = 0.0661 M and [CO] = 3.85 M

18.2 Section Quiz. 1. In a reaction at equilibrium, reactants and products a) decrease in concentration. b) form at equal rates. c) have equal concentrations. d) have stopped reacting.

18.2 Section Quiz. 2. In the reaction 2NO2(g)  2NO(g) + O2(g), increasing the pressure on the reaction would cause a) the amount of NO to increase. b) the amount of NO2 to increase. c) nothing to happen. d) the amount of O2 to increase.

18.2 Section Quiz. 3. Changing which of the following would NOT affect the equilibrium position of a chemical reaction? a) concentration of a reactant only b) concentration of a product only c) temperature only d) volume only

18.2 Section Quiz. 4. For the following reaction, Keq = 1. A(g) + B(g)  C(g) + D(g) Therefore, at equilibrium a) [C] = [A]. b) [A][B] = 0. c) [AB] = [CD] = 1. d) [A][B] = [C][D].

18.3 Solubility Equilibrium

The Solubility Product Constant 18.3 The Solubility Product Constant The solubility product constant (Ksp), equals the product of the concentrations of the ions, each raised to a power equal to the coefficient of the ion in the dissociation equation. The smaller the numerical value, the lower the solubility of the compound.

18.3

18.3

Sample Problem 18.3 The chromate ion is responsible for the brilliant yellow color of lead(II)chromate.

for Sample Problem 18.3

Practice Calculate the solubility in mol/L of copper(II) carbonate. Ksp = 2.5 x 10-10 at 298 K. The Ksp of lead (II) carbonate is 7.40 x 10-14 at 298 K. What is the solubility of lead carbonate in g/L?

Example Magnesium hydroxide is a white solid obtained from seawater and used in many medications. Determine the hydroxide ion concentration in a saturated solution of Mg(OH)2 at 298 K. Ksp = 5.6 x 10-12

Practice 1) Calculate [Ag+] in a solution of AgBr (Ksp = 5.4 x 10-13) 2) Calculate [F-] in a solution of CaF2 (Ksp = 3.5 x 10-11) 3) Calculate the solubility of Ag3PO4 (Ksp = 2.6 x 10-18)

Practice 4) The solubility of silver chloride (AgCl) is 1.86 x 10-4 g/100 g of H2O at 298 K. Calculate Ksp for AgCl.

The Common Ion Effect The Common Ion Effect 18.3 The Common Ion Effect The Common Ion Effect How can you predict whether precipitation will occur when two salt solutions are mixed? A precipitate of barium sulfate forms as barium nitrate (Ba(NO3)2) and sodium sulfate (Na2SO4) solutions are mixed. Applying Concepts What is the product of the concentrations of barium ion and sulfate ion after precipitation is complete?

18.3 The Common Ion Effect If the product of the concentrations of two ions in the mixture is greater than the Ksp of the compound formed from the ions, a precipitate will form.

18.3 The Common Ion Effect A common ion is an ion that is found in both salts in a solution. The lowering of the solubility of an ionic compound as a result of the addition of a common ion is called the common ion effect.

Sample Problem 18.4

for Sample Problem 18.4

18.3 Section Quiz. 2.0  10-17M 8.9  10-26M 8.9  10-25M 2.0  1017M 1. What is the concentration of a saturated solution of silver sulfide? The Ksp of Ag2S is 8.0  10-51. 2.0  10-17M 8.9  10-26M 8.9  10-25M 2.0  1017M

18.3 Section Quiz. 2. Adding which of these solutions to a saturated solution of BaSO4 will cause the solubility of BaSO4 to decrease? I. BaCl2(aq) II. Na2SO4 (aq) (I) only (II) only (I) and (II) neither solution

18.3 Section Quiz. 3. The Ksp of AgBr is 5.0  10-13. When 7.1  10-6 mol/L solutions of NaBr(aq) and AgNO3(aq) are mixed, we would expect no precipitate to form. a definite precipitation reaction. no reaction. a saturated solution but no visible precipitation.

18.3 Section Quiz. 4. After the common ion effect causes a precipitate to form in a solution, the solution will no longer be saturated. the solution will again be saturated. the solution will be supersaturated. there will be no solute left in the solution.

18.4 Entropy and Free Energy

Free Energy and Spontaneous Reactions 18.4 Free Energy and Spontaneous Reactions A spontaneous reaction occurs naturally and favors the formation of products at the specified conditions. Cadmium sulfide forms spontaneously when solutions of sodium sulfide and cadmium nitrate are mixed. Inferring Is free energy released in this reaction?

18.4 A nonspontaneous reaction is a reaction that does not favor the formation of products at the specified conditions. Photosynthesis is a nonspontaneous reaction that requires an input of energy. Many non-spontaneous reactions are required for a plant to grow. Nonspontaneous reactions can occur when they are coupled with spontaneous reactions. Inferring What is another requirement for this photosynthesis reaction?

Free Energy and Spontaneous Reactions 18.4 Free Energy and Spontaneous Reactions Spontaneous reactions produce substantial amounts of products at equilibrium and release free energy. Free energy is energy that is available to do work.

Entropy Entropy is a measure of the disorder of a system. 18.4 Entropy Entropy is a measure of the disorder of a system. Physical and chemical systems attain the lowest possible energy. The law of disorder states that the natural tendency is for systems to move in the direction of maximum disorder or randomness.

18.4 Entropy An increase in entropy favors the spontaneous chemical reaction; a decrease favors the nonspontaneous reaction.

18.4 For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid. Entropy is a measure of the disorder of a system. a) For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid.Thus entropy increases in reactions in which solid reactants form liquid or gaseous products. Entropy also increases when liquid reactants form gaseous products. b) Entropy increases when a substance is divided into parts. For instance, entropy increases when a crystalline ionic compound, such as sodium chloride, dissolves in water. This is because the solute particles— sodium ions and chloride ions—are more separated in solution than they are in the crystal form. c) Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules. d) Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder.

Entropy Entropy increases when a substance is divided into parts. 18.4 Entropy is a measure of the disorder of a system. a) For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid. Thus entropy increases in reactions in which solid reactants form liquid or gaseous products. Entropy also increases when liquid reactants form gaseous products. b) Entropy increases when a substance is divided into parts. For instance, entropy increases when a crystalline ionic compound, such as sodium chloride, dissolves in water. This is because the solute particles— sodium ions and chloride ions—are more separated in solution than they are in the crystal form. c) Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules. d) Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder.

18.4 Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules. Entropy is a measure of the disorder of a system. a) For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid. Thus entropy increases in reactions in which solid reactants form liquid or gaseous products. Entropy also increases when liquid reactants form gaseous products. b) Entropy increases when a substance is divided into parts. For instance, entropy increases when a crystalline ionic compound, such as sodium chloride, dissolves in water. This is because the solute particles— sodium ions and chloride ions—are more separated in solution than they are in the crystal form. c) Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules. d) Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder.

18.4 Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder. Entropy is a measure of the disorder of a system. a) For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid. Thus entropy increases in reactions in which solid reactants form liquid or gaseous products. Entropy also increases when liquid reactants form gaseous products. b) Entropy increases when a substance is divided into parts. For instance, entropy increases when a crystalline ionic compound, such as sodium chloride, dissolves in water. This is because the solute particles— sodium ions and chloride ions—are more separated in solution than they are in the crystal form. c) Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules. d) Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder.

Practice Predict the sign of ΔS: ClF(g) + F2(g) → ClF(g) NH3(g) → NH3(aq) CH3OH(l) → CH3OH(aq) C10H8(l) → C10H8(s)

Enthalpy, Entropy, and Free Energy 18.4 Enthalpy, Entropy, and Free Energy The size and direction of enthalpy changes and entropy changes together determine whether a reaction is spontaneous; that is, whether it favors products and releases free energy.

Enthalpy, Entropy, and Free Energy 18.4 Enthalpy, Entropy, and Free Energy

18.4 Gibbs Free-Energy The Gibbs free-energy change is the maximum amount of energy that can be coupled to another process to do useful work. The numerical value of ΔG is negative in spontaneous processes because the system loses free energy.

Example For a process, ΔH = 145 kJ and ΔS = 322 J/K. Is the process spontaneous at 382 K?

Practice Determine whether the reaction is spontaneous A) ΔH = -75.9 kJ; ΔS = 138 J/K; T = 273 K B) ΔH = -27.6 kJ; ΔS = -55.2 J/K; T = 535 K C) ΔH = 365 kJ; ΔS = -55.2 J/K; T = 388 K

Practice Given ΔH = -144 kJ and ΔS = -36.8 J/K for a reaction, determine the lowest temperature in kelvins at which the reaction would be spontaneous.

18.4 Section Quiz. 1. Free energy from a reaction is the amount of energy that is absorbed by an entropy decrease. equal to the enthalpy change. wasted as heat. available to do work.

18.4 Section Quiz. 2. Free energy is always available from reactions that are endothermic. nonspontaneous. at equilibrium. spontaneous.

18.4 Section Quiz. 3. Choose the correct words for the spaces: Spontaneous reactions produce ________ and substantial amounts of _________ at equilibrium. free energy, products no free energy, reactants free energy, reactants no free energy, products

18.4 Section Quiz. 4. Which of the following involves a decrease in entropy? Natural gas burns. A liquid freezes. Dry ice sublimes. Water evaporates.

18.4 Section Quiz. 5. A reaction is spontaneous if enthalpy decreases and entropy increases. enthalpy increases and entropy increases. enthalpy decreases and entropy decreases. enthalpy increases and entropy decreases.

18.4 Section Quiz. 6. Choose the correct words for the spaces: Gibbs free-energy change is the _________ amount of energy that can be ___________ another process to do useful work. maximum, coupled to maximum, duplicated by spontaneous, coupled to minimum, duplicated by